This invention relates generally to liquid pumping systems in which the damping of pressure pulses in a high-pressure flow line is required.
A particular application of this invention is a high-pressure liquid chromatography system in which a carrier liquid containing a quantity of sample solution to be analyzed is sent to a chromatographic column in pulses by the action of a reciprocating pump. In order to provide a continuous flow of liquid to the chromatographic column, it is necessary to insert a pulse damper in the flow line between the pump and the chromatographic column.
In the prior liquid chromatography art, a block of plastic material such as polytetrafluoroethylene (available commercially under the Teflon trademark) was placed in the flow line between a reciprocating pump and a chromatographic column to serve as a pulse damper. Since a Teflon block is compressible, it can store energy by becoming compressed during that portion of the pumping cycle when the pressure in the flow line is rising, and can release the stored energy by decompressing when the pressure in the flow diminishes. Thus, over a complete pumping cycle, the pressure pulses generated in the flow line liquid by the reciprocating pump can be smoothed out before the flow line liquid reaches the chromatographic column. A pulse damper of this kind was described in U.S. Pat. No. 4,024,061.
With such prior art pulse damping devices in which a compressible body was disposed directly in the flow line, the choice of materials for the compressible body was generally limited to those plastics that are chemically inert with respect to the kinds of fluids used in liquid chromatography. Only a few plastic materials, such as polytetrafluoroethylene (Teflon) and polychlorotrifluoroethylene (Kel-F), can meet this requirement. Furthermore, the damping characteristics of a compressible plastic body are not adjustable. Thus, with the prior art pulse damping devices that utilized the compressibility of a plastic body as the pulse damping mechanism, it was not possible to match the damping characteristics of the pulse damper with the pulsing characteristics of the pump except by changing the volume of the compressible plastic material.
Also, with prior art pulse dampers having a compressible plastic body in a housing structure coupled to the flow line, unflushable "dead volumes" tended to occur in portions of the housing structures as the flow line fluid altered its course through the housing structure because of the obstacle provided by the compressible plastic body. Minute quantities of the flow line fluid tended to become trapped in such dead volumes, thereby becoming a source of contamination for the chromatographic system.
The compressibility of polytetrafluoroethylene at low pressures (i.e., in the range from atmospheric pressure to about 1000 psi) was well-known to the prior art. Apparently, the assumption also prevailed in the prior art that polytetrafluoroethylene would exhibit substantially similar compressibility characteristics at high pressures (i.e., above 1000 psi), as evidenced by the fact that pulse dampers utilizing the compressibility of a polytetrafluoroethylene body disposed directly in the flow line were used in the prior art even in high-pressure liquid chromatography applications. However, experimental work that resulted in the present invention has surprisingly revealed that the compressibility of polytetrafluoroethylene decreases quite rapidly with increasing pressure at pressures above 1000 psi. Consequently, the effectiveness of prior art pulse dampers at pressures above 1000 psi becomes severely reduced as flow line pressures increased.